Twisters are not increasing in numbers but they are clustering more often, a bizarre pattern that has meteorologists stumped

While the United States has not experienced an overall increase in tornadoes over the last several decades, more twisters are now grouping together, according to decades’ worth of tornado data analyzed by the National Oceanic and Atmospheric Administration. If the trend continues, U.S. residents could see even fewer tornado days in the coming years, but many of those days could pack a punch.

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Scientists have been concerned that the atmospheric warming from climate change could somehow affect the frequency or intensity of the violent tornadoes that plague much of the United States. But it hasn’t been clear how those changes would manifest.

“We know that tornadoes form when there is lots of energy available for thunderstorms and when there is lots of wind shear,” says NOAA tornado researcher Harold Brooks. Wind shear is the change in the wind’s speed or direction as you go higher in the atmosphere, and strong shear helps give a tornado its twist. Global warming is increasing the energy available for storms to form, but it’s also expected to decrease wind shear, Brooks notes.

To see how climate change might be affecting tornadoes, scientists need to look at their historical patterns. That can be difficult, in part because there is no traditional season for tornadoes as there is for hurricanes. Twisters have struck on every calendar day of the year within the past six decades. Further complicating matters, the way twisters are observed and reported has changed over time. Scientists know that those observational differences have changed the numbers of the smallest tornadoes—those rated F0 on the Enhanced Fujita scale. These storms have increased from about 100 per year in the 1950s to some 800 annually today. Larger storms—F1 to F5—have stayed constant, numbering around 500 on average yearly, although their frequency can vary widely from year to year.

In the new study, published today in Science, Brooks and his colleagues tallied U.S. storms from 1954 to 2013, leaving out the small F0 twisters. Then they looked at the days on which those storms occurred. They found that the frequency of tornado days has declined over that time. In 1973, for instance, tornadoes formed on 187 days. By contrast, 2011 saw twisters on only 110 days—but nine of those days saw more than 30 tornadoes each.

“In effect, there is a low probability of a day having a tornado, but if a day does have a tornado, there is a much higher chance of having many tornadoes,” the researchers write. Now, about a fifth of a year’s cyclones occur on just three days of that year.

The NOAA results are similar to those of another study, published earlier this year in Climate Dynamics, that also found an increase in tornado density—twisters are clustering both in time and space. “Since we both used the same data, it is not surprising that the conclusions are the same,” says that study’s lead author James Elsner of Florida State University in Tallahassee. “It is a bit surprising to me that they do not offer speculation on the possible cause.”

The NOAA researchers are reluctant to attribute the change in tornado timing to any cause at this point, though they do not think it has anything to do with how the storms are reported. “We need to look at the distribution of favorable [tornado] conditions on small time and space scales and see how those have changed over the years, if they have changed,” Brooks says. Global climate change isn’t the only factor that may be affecting tornado patterns. Brooks says researchers should also consider changes in land-use patterns, for instance, because vegetation can affect local weather and microclimates.

But Elsner thinks that climate is probably involved. “The greater heat and moisture in the atmosphere is a direct result of a warming planet, and the warming is greater at the poles than at lower latitudes, amplifying and slowing the jet stream,” he says. That provides sufficient wind shear for the tornadoes. “Shear will decrease on average across the globe as the warming in the Arctic outpaces warming elsewhere, but sufficient shear persists regionally when the jet stream waves amplify and stall,” he says. And that could lead to clustered tornadoes.

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About Sarah Zielinski

Sarah Zielinski is an award-winning science writer and editor. She is a contributing writer in science for Smithsonian.com and blogs at Wild Things, which appears on Science News.

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